Polyaniline and the like are known materials as a conductive polymer. In addition to its electrical properties, polyaniline has an advantage that it can be synthesized relatively easily from inexpensive aniline and exhibits excellent stability to air or the like in the state that it shows conductivity.
As the method for producing polyaniline, a method in which aniline or an aniline derivative is subjected to oxidative electro-polymerization or chemical oxidative polymerization is known.
As for the oxidative electro-polymerization method, a method in which aniline is polymerized on electrodes is disclosed in Patent Document 1 or Patent Document 2. In the oxidative electro-polymerization, a film having excellent electrical properties is obtained. However, in general, as compared with the chemical oxidative polymerization, the production cost is high in the oxidative electro-polymerization, and hence, the oxidative electro-polymerization is not suited to the large-scale production. In addition, it is difficult to obtain a formed article which has a complicated shape by this method.
On the other hand, in order to obtain a conductive polymer of aniline or an aniline derivative by the chemical oxidative polymerization, a step of protonation in which a dopant (doping agent) is added to polyaniline in the non-conductive base state (the so-called Emeraldine base state) is required.
However, since polyaniline in the non-conductive base state is hardly dissolved in most of organic solvents, it is not suited to the production on the industrial scale. In addition, conductive polyaniline (the so-called Emeraldine base state) generated after protonation is substantially insoluble and non-melting. Therefore, it is difficult to produce a conductive composited material and a formed article thereof easily.
Under such circumstances, as the method for improving doping of polyaniline in the non-conductive base state and affinity of conductive polyaniline after doping for an organic solvent, several proposals were made.
For example, Non-Patent Document 1 states that, by using a protonic acid which has affinity for an organic solvent such as dodecylbenzene sulfonic acid and camphorsulfonic acid (CSA) as the dopant, excellent electrical properties are exhibited.
Patent Document 3 states a method in which polyaniline in the non-conductive base state (adamantane sulfonic acid, for example) is used as a dopant, and this adamantane sulfonic acid is dissolved in m-cresol.
Non-Patent Document 2 discloses a method in which polyaniline in the non-conductive base state is doped in a specific solvent such as 2,2-dichloroacetic acid (halogen-based strong acid) using 2-acrylamide-2-methyl-propanesulfonic acid as a dopant, for example.
As in the case of Patent Document 2, Patent Document 4 discloses a method in which 2,2-dichloroacetic acid is used as a solvent and polyaniline in the non-conductive base state is doped using di(2-ethylhexyl)ester of sulfosuccinic acid as a dopant.
However, a formed article formed of conductive polyaniline obtained by a method disclosed in Patent Documents 1 to 4 and Non-Patent Documents 1 and 2 is not necessarily excellent in electric properties such as electric conductivity.
In order to increase conductivity of a formed article formed of conductive polyaniline, Patent Document 5 discloses a conductive polyaniline composition comprising (a) a substituted or unsubstituted polyaniline composite material which has been protonated; and (b) a conductive polyaniline composition comprising a compound having a phenolic hydroxyl group, which are dissolved in an organic solvent which is substantially immiscible with water. Further, Patent Document 5 discloses that the amount of the (b) phenolic compound is normally 0.01 to 1000 mass %, preferably 0.5 to 500 mass %, relative to the (a) protonated substituted or unsubstituted polyanilinecomposite material, and discloses a method in which sodium diisooctylsulfosuccinic acid and aniline are dissolved in toluene, hydrochloric acid is added to the resulting solution, the flask is cooled on ice water bath, and a solution obtained by dissolving ammonium persulfate in hydrochloric acid was added dropwise to conduct polymerization of aniline.
In Patent Document 5, aniline is polymerized in the presence of hydrochloric acid. The reason therefor is that higher conductivity can be obtained when conductive polyaniline is synthesized by polymerizing aniline in the presence of aniline. Further, Non-Patent Document 3 states that conductive polyaniline having a high molecular weight cannot be obtained unless aniline is polymerized in the presence of hydrochloric acid. In general, in a polymer, there is a close correlation between the molecular weight and the physical properties of a material. For example, regarding the strength of a film which is formed, a film formed from a polymer having a higher molecular weight has a higher strength. As mentioned above, in the case of polyaniline, when polymerization is conducted by using hydrochloric acid, it is possible to obtain a material which is industrially advantageous. However, in the field of electronic components, since a metal part may corrode when a material containing chloride is used or because of tightening of global environmental regulations in recent years, chlorine-free polyaniline has been demanded.